Remove useless / ghost code

This PR removes useless explicit setting of `@as` to nil in SingletonResource initializer.

Since `@as` ivar is set to be equal to `options[:as]` right after
there no need to set `@as` variable to `nil` value explicitly.

If `options[:as]` is present `@as` will take this value otherwise
it will be set to `nil`.

I think this line comes from an old version that was left behind.